The present invention relates to an optical window suitable for use in radiation position-sensitive detectors and other devices.
Optical windows are known that are to be coupled to a scintillator and a photomultiplier tube for use in such devices as a radiation position-sensitive detector.
FIG. 7 is a plan view of an optical fiber plate window which is commonly known as a typical example of such optical windows. The window generally indicated as 50 is formed of a lattice pattern of optical fibers each having a coaxial structure of core glass 51 and clad glass 52. The core glass 51 has a greater refractive index than the clad glass 52. The optical fiber plate window 50 accepts only light that is incident at angles within the range of its "numerical aperture" and allows total reflection of light at the boundary between the core glass 51 and the clad glass 52 so that the light will propagate through the fiber guide without reflection loss.
In the optical fiber plate window 50 shown in FIG. 7, the core glass 51 and the clad glass 52 have to be formed of dissimilar materials having different refractive indices, so it is difficult to eliminate the thermal expansion mismatch between the two members. One of the problems associated with this difficulty has been the high probability that cracking will occur due to a temperature change or other factors during the fabrication or practical use of the optical window 50.
In addition, because of the circular cross section of individual optical fibers, a gap 53 forms between adjacent fibers and light entering this gap will be directly transmitted through it, producing spatial divergence of light to become a potential cause of crosstalk. An absorber may be provided in the gap 53 in order to prevent the occurrence of such crosstalk, but then the light entering the gap 53 will be lost through absorption by the absorber.
As already mentioned, the optical fiber plate window 50 has its own numerical aperture, which may be increased when light is to be accepted from the aerial layer. However, when the window 50 is to be coupled with a scintillator or the like in practical applications, an optical coupler is customarily inserted between the scintillator and the window 50 so as to prevent reflection loss at the boundary between these two members. In the presence of the optical coupler, there occurs a corresponding decrease in the numerical aperture of the window to cause increased loss of light in it.